In radio transmission, it is necessary to send audio signal e.g. music, speech, etc. from a broadcasting station over great distances to a receiver. This communication of audio signal does not employ any wire and is sometimes called wireless. The audio signal cannot be sent directly over the air for appreciable distance. Even if we convert the audio signal into electrical signal, the latter cannot be sent very far without employing large amount of power. The energy of a wave is directly proportional to its frequency. At audio frequency 20 Hz to 20 kHz, the signal power is quite small and radiation is not practicable.
The radiation of electrical energy is practicable only at high frequencies e.g. above 20 kHz. The high frequency signals can be sent thousands of miles even with comparatively small power. Hence, if audio signal is to be transmitted properly, we must devise ways that will permit transmission to occur at high frequencies while simultaneously allowing the carrying of the audio signal. This achieved by superimposing electrical audio signal on high frequency carrier. The resultant waves are known as modulated waves or radio waves and the process is called modulation. At the receiver, the audio signal is extracted from the modulated wave by the process called demodulation.
The signal is then amplified and reproduced into sound by the loudspeaker.
Related: Basics of Radio Frequency (RF) and Wireless Communication Systems
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